1. Field of the Invention
The present invention relates to optical measurement of the waveform of target light and more particularly, to a method of measuring the waveform of target light and an apparatus for measuring the same, which are applicable to measurement of the waveform of ultra-high speed pulsed light used for optical communication and/or optical information processing.
2. Description of the Related Art
In recent years, the capacity of data to be transmitted in optical communications systems has been increasing rapidly and accordingly, not only the techniques for the wavelength multiplexing method that transmits the data using different wavelengths of signal light but the techniques for raising the data transmission rate in each wavelength to 100 Gb/s or higher have been being researched and developed actively. Under such circumstances, there have been the increasing need to develop the techniques for generating stable, coherent, ultra-high speed optical pulses and to measure the waveform of the ultra-high speed optical pulse train in real time with sufficiently high time resolution. In particular, the xe2x80x9ceye pattern measurementxe2x80x9d that measures directly an optical pulse train modulated by random bit data is essential to evaluate the characteristics of optical transmission systems.
A typical one of the known methods of measuring optical pulse trains is to use a ultra-high speed photoelectric converter and an electrically sampling oscilloscope. In this case, the xe2x80x9ceye pattern measurementxe2x80x9d can be performed, but in the present circumstances, the higher end of the measurable frequency range of light is, at most, approximately 40 GHz. As a result, it is difficult to measure the waveform of ultra-high speed optical pulse trains having a data transmission rate that exceeds about 40 Gb/s in each wavelength in real time with sufficiently high time resolution.
To solve the above-described difficulty, a method of measuring the waveform of target light has been developed and actually used. In this method, pulsed target light to be measured and pulsed sampling light having a sufficiently narrower pulse width than the target light is supplied to a specific nonlinear optical member, thereby generating intensity cross-correlated light between the target light and the sampling light due to nonlinear optical effects. On the basis of the cross-correlated light thus generated, the waveform of the target light is measured. In this method, the target light can be optically sampled and therefore, the above-described difficulty can be solved. Specifically, the waveform of ultra-high speed optical pulses having a data transmission rate that exceeds about 40 Gb/s in each wavelength can be measured in real time with sufficiently high time resolution.
Examples of the prior-art apparatuses of this type for measuring the waveform of target light pulses using the above-described method are disclosed in the Japanese Non-Examined Patent Publication No. 8-29814 published in 1996 and the Japanese Non-Examined Patent Publication No. 9-160082 published in 1997. FIG. 1 shows a typical one of the prior-art apparatuses of this type, in which thick lines with arrows indicate the flow of optical signals while thin lines with arrows indicate the flow of electrical signals.
The prior-art measuring apparatus 200 comprises a driving signal oscillator 262, a sampling light source 263, a nonlinear optical member 264, an optical filter 265, an optical detector 266, an electrical signal processing circuit 267, and a display device 268. The apparatus 200 itself is electrically and optically connected to an external apparatus 261.
The external apparatus 261 includes a driving signal oscillator 271 that oscillates an electrical driving signal SD1 with a frequency f0 and a target light source 272 that is driven by the oscillator 271 to emit pulsed target light LT0. The target light LT0 thus emitted has a repetition frequency equal to the frequency f0 of the driving signal SD1. An example of the waveform of the target light LT0 is shown by the waveform a in FIG. 14.
The oscillator 262, which is electrically connected to the oscillator 271 provided in the external apparatus 261, oscillates a driving signal SD2 having a frequency fS synchronized in phase with the driving signal SD1 having the frequency f0. The reason why the oscillator 262 is electrically connected to the oscillator 271 is to synchronize the phase of the target light LT0 with the phase of the sampling light LTS. Because of the phase synchronization between the light LT0 and LTS, the fluctuation of time difference xcex4t of each pulse of the target light LT0 from each pulse of the sampling light LTS, (i.e., mutual jitter), is decreased. Thus, the time resolution can be prevented from degrading. In principle, possible time resolution is approximately equal to the pulse width of the sampling light LTS.
The sampling light source 263 is driven by the driving signal oscillator 262, emitting the pulsed sampling light LTS. The sampling light LTS thus emitted has a repetition frequency fS, where fS=(f0/N)xe2x88x92xcex94f, f0 is the repetition frequency of the target light LT0, xcex94f is a frequency difference, and N is a natural number (i.e., N=1, 2, 3, 4, . . .). The repetition frequency fS of the sampling light LTS is slightly different by xcex94f from the divided frequency of the target light LT0 by N, i.e., (f0/N). For example, when N=1, the sampling light LTS has a waveform b shown in FIG. 14. In this case, each pulse of the sampling light LTS has a time difference xcex4t from the corresponding pulse of the target light LT0.
The target light LT0 and the sampling light LTS thus generated enters the nonlinear optical member 264, emitting intensity cross-correlated light LTCC between the light LT0 and LTS thus supplied.
The nonlinear optical member 264 may be made of a ferroelectric crystal such as KTP (KTiOPO4) that causes a secondary nonlinear optical effect, e.g., the Sum Frequency Generation (SFG). Alternately, the member 264 maybe formed by a semiconductor optical amplifier or a quartz-system optical waveguide such as an optical fiber that causes a tertiary nonlinear optical effect, e.g., the Four Wave Mixing (FWM). The member 264 is used to emit the intensity cross-correlated light LTCC between the pulses of the target light LT0 and the sampling light LTS. For example, the cross-correlated light LTCC thus emitted has a waveform shown by the waveform c in FIG. 14. The cross-correlated light LTCC has a repetition frequency equal to the repetition frequency fS of the sampling light LTS.
Here, the time difference xcex4t of the pulse of the sampling light LTS from the corresponding pulse of the target light LT0 corresponds to the sampling time. Thus, it is expressed by the following equation (1).                               δ          ⁢                      xe2x80x83                    ⁢          t                =                                            1                              f                s                                      -                          N                              f                0                                              ≅                                    Δ              ⁢                              xe2x80x83                            ⁢              f                                      f              s              2                                                          (        1        )            
For example, when the repetition frequency fS of the sampling light LTS is set as 1 GHz and the frequency difference xcex94f is set as 100 kHz, the time difference xcex4t is given as 0.1 ps (picosecond) by the equation (1).
The optical filter 265 removes the target light LT0 and the sampling light LTS and their secondary and higher harmonics (which serve as background light LTB of the intensity cross-correlated light LTCC), allowing only the cross-correlated light LTCC to pass through the filter 265.
The optical detector 266 photoelectrically converts the cross-correlated light LTCC thus passed through the filter 265 to generate a pulsed electrical signal SCC. The signal SCC is supplied to the signal processing circuit 267.
The detector 266 needs to have a frequency band equal to or higher than the repetition frequency fS of the sampling light LTS. This is due to the fact that each pulse of the cross-correlated light LTCC needs to be photoelectrically converted separately in such a way as to cause no interference with its adjoining pulses in order to display correctly an eye pattern shown by the waveform d in FIG. 14 in the display device 268. However, for example, even if the repetition frequency f0 of the target light LT0 is 100 GHz, the repetition frequency fS of the sampling light LTS can be lowered to approximately 100 MHz by setting the dividing factor N as 1000 in the sampling light source 263. Thus, it is sufficient for the optical detector 266 to have a frequency band of approximately 100 MHz.
The electrical signal processing circuit 267 samples the peak value of the pulsed electrical signal SCC outputted by the detector 266 in synchronization with the repetition (i.e., sampling) frequency fS of the sampling light LTS. Thus, the circuit 267 generates an electrical peak signal SCCP that represents the peak values of the electrical signal SCC and supplies the same to the display device 268. In FIG. 14, the peak values of the electrical signal SCC are shown by circular dots or spots in the vicinity of the waveform c.
The display device 268 displays the waveforms so as to be overlapped with each other on its screen at a period of [1/(Nxc2x7xcex94f)] on the basis of the electrical peak signal SCCP outputted by the signal processing circuit 267. Thus the device 268 displays the eye pattern as shown by the waveform d.
The measuring person or tester evaluates the characteristics of the optical transmission system according to the opening level of the eye pattern displayed on the screen of the display device 263.
As explained above, with the prior-art measured apparatus 200 shown in FIG. 1, to synchronize the phase of the sampling light LTS with the phase of the target light LT0, the driving signal oscillator 271 provided in the external apparatus 261 is electrically connected to the driving signal oscillator 262 provided near the sampling light source 263. However, for example, when the ultra-high speed pulses of the target light LT0 having a repetition frequency that exceeds 40 Gb/s, which has been emitted in the external apparatus 261 and transmitted therefrom by way of an optical transmission path (e.g., optical fibers) of several kilometers in length, is measured in real time, it is not realistic to electrically interconnect the oscillator 262 with the oscillator 271 by way of cable of several kilometers in length. This is due to the following reason.
It is known that any ultra-high speed electrical signal is unable to be transmitted over a very long distance by way of cable due to attenuation of the electrical signal. Taking this problem into consideration, the initial electrical signal is converted to an optical signal, the optical signal thus converted is transmitted through an optical transmission path, and then, the optical signal thus transmitted is reconverted to a resultant electrical signal on purpose. From this point of view, it is not meaningless to electrically interconnect the oscillator 262 with the oscillator 271 by way of cable of several kilometers in length. This means that this is not realistic for practical use.
Thus, with the prior-art apparatus 200, because of the necessity to electrically interconnect the two oscillators 271 and 262 with each other, it is unable to substantially synchronize the phase of the target light LT0 with the phase of the sampling light LTS if the oscillators 271 and 262 are located far away from each other. As a result, there is a problem that the fluctuation of the time difference xcex4t (i.e., the mutual jitter) is not decreased, thereby degrading the time resolution.
Additionally, an apparatus and method for measuring the waveform of target light are disclosed in the Japanese Non-Examined Patent Publication No. 9-138165 published in 1997, in which sampling light with the repetition frequency that follows the fluctuation of repetition frequency of the target light is generated. In this apparatus and method, the nonlinear optical effects between the target light and the sampling light are used to measure the waveform of the target light and to control the repetition frequency of the sampling light.
However, these apparatus and method of the Publication No. 9-138165 does not refer to the problem of the degradation of the time resolution in measurement.
Accordingly, an object of the present invention is to provide a method and an apparatus for measuring she waveform of light that make it possible to synchronize easily the phase of sampling light with the phase of target light even if the target light is in the form of ultra-high speed pulses (e.g., 40 Gb/s or higher in repetition frequency) and is transmitted by way of a long transmission path (e.g., several kilometers in length).
Another object of the present invention is to provide a method and an apparatus for measuring the waveform of light that make it possible to measure the waveform of target light with sufficient time resolution in real time even if the target light is in the form of ultra-high speed pulses (e.g., 40 Gb/s or higher in repetition frequency) and is transmitted by way of a long transmission path (e.g., several kilometers in length).
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a method of measuring a pulse waveform of target light is provided, which comprises the steps of:
(a) generating sampling light having a pulse width narrower than that of target light from the target light;
a repetition frequency of the sampling light having a constant difference with respect to a repetition frequency of the target light;
(b) supplying the sampling light and the target fight to a nonlinear optical member to generate cross-correlated light between the sampling light and the target light; and
(c) measuring a waveform of the target light based on the cross-correlated light.
With the method according to the first aspect of the present invention, in the step (a), the sampling light having a pulse width narrower than that of the target light is generated from the target light. In the step (b), the sampling light and the target light are supplied to the nonlinear optical member to generate the cross-correlated light between the sampling light and the target light. In the step (c), the waveform of the target light is measured based on the cross-correlated light. Moreover, the repetition frequency of the sampling light has the constant difference with respect to the repetition frequency of the target light.
As a result, the phase of the sampling light can be easily synchronized with the phase of the target light even if the target light is ultra-high speed pulsed light and is transmitted by way of a long transmission channel. Thus, the waveform of the target light can be measured with sufficient time resolution in real time.
In a preferred embodiment of the method according to the first aspect, the step (a) of generating the sampling light comprises the substeps of:
(a-1) extracting clock light from the target light;
the clock light being synchronized with the target light;
(a-2) generating an electrical clock signal from the clock light;
(a-3) generating an electrical driving signal in such a way that a constant frequency difference exists between a frequency of the electrical driving signal and a frequency of the electrical clock signal; and
(a-4) generating the sampling light based on the electrical driving signal.
In another preferred embodiment of the method according to the first aspect, in the substep (a-1) of extracting the clock light from the target light, the target light is supplied to a passive mode-locked laser, thereby generating the clock light. The clock light has a repetition frequency (1/N) times as much as a repetition frequency of the target light, where N is a natural number.
It is preferred in this preferred embodiment that a passive mode-locked semiconductor laser is used as the passive mode-locked laser.
In this preferred embodiment of the method according to the first aspect, in the substep (a-3) of generating the electrical driving signal,
an electrical offset signal is generated by frequency-dividing the clock signal;
an electrical difference-frequency signal is generated by frequency-mixing the electrical offset signal and the electrical difference-frequency signal together; and
the electrical driving signal is generated based on a phase difference between the electrical offset signal and the electrical difference-frequency signal.
Preferably, there may be additionally provided with the steps of:
(d) generating an electrical cross-correlated signal from the cross-correlated light;
(e) sampling peak values of the electrical cross-correlated signal; and
(f) displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
Preferably, the step (e) of sampling the peak values of the electrical cross-correlated signal is performed to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.
In still another preferred embodiment of the method according to the first aspect, in the substep (a-3) of generating the electrical driving signal,
an electrical offset signal is generated by frequency-dividing the clock signal;
an electrical difference-frequency signal is generated by frequency-mixing the electrical offset signal and the electrical clock signal together; and
the electrical driving signal is generated based on a phase difference between the electrical driving signal and the electrical difference-frequency signal.
Preferably, there are additionally provided with the steps of:
(d) generating an electrical cross-correlated signal from the cross-correlated light;
(e) sampling peak values of the electrical cross-correlated signal; and
(f) displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
The step (e) of sampling the peak values of the electrical cross-correlated signal is preferably performed to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.
In a further preferred embodiment of the method according to the first aspect, in the substep (a-3) of generating the electrical driving signal,
an electrical offset signal is generated by frequency-dividing the clock signal;
an electrical, additional driving signal is generated by frequency-dividing the electrical driving signal; and
the electrical driving signal is generated based on a phase difference between the electrical offset signal and the electrical, additional driving signal.
In this embodiment, preferably, there are additionally provided with the steps of:
(d) generating an electrical cross-correlated signal from the cross-correlated light;
(e) sampling peak values of the electrical cross-correlated signal; and
(f) displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
Preferably, the step (e) of sampling the peak values of the electrical cross-correlated signal is performed to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.
According to a second aspect of the present invention, an apparatus for measuring a pulse waveform of target light is provided, which comprises:
(a) a sampling light generator for generating sampling light having a pulse width narrower than that of target light from the target light;
a repetition frequency of the sampling light having a constant difference with respect to a repetition frequency of the target light;
(b) a cross-correlated light generator for generating a cross-correlated light between the sampling light and the target light by supplying the sampling light and the target light to a nonlinear optical member; and
(c) a measuring device for measuring a waveform of the target light based on the cross-correlated light.
With the apparatus according to the second aspect of the present invention, because of the same reason as explained in the method according to the first aspect, the phase of the sampling light can be easily synchronized with the phase of the target light even if the target light is ultra-high speed pulsed light and is transmitted by way of a long transmission path. Thus, the waveform of the target light can be measured with sufficient time resolution in real time.
In a preferred embodiment of the apparatus according to the second aspect, the sampling light generator (a) comprises:
(a-1) a clock light extractor for extracting clock light from the target light;
the clock light being synchronized with the target light;
(a-2) an electrical clock signal generator for generating an electrical clock signal from the clock light;
(a-3) an electrical driving signal generator for generating an electrical driving signal in such a way that a constant frequency difference exists between a frequency of the electrical driving signal and a frequency of the electrical clock signal; and (a-4) a sampling light generator for generating the sampling light based on the electrical driving signal.
In this embodiment, preferably, the clock light extractor (a-1) comprises a passive mode-locked laser;
the target light being supplied to the passive mode-locked laser, thereby generating the clock light;
the clock light having a repetition frequency (1/N) times as much as a repetition frequency of the target light, where N is a natural number.
A passive mode-locked semiconductor laser is preferably used as the passive mode-locked laser.
In another preferred embodiment of the apparatus according to the second aspect, the electrical driving signal generator (a-3) comprises:
an electrical offset signal generator for generating an electrical offset signal by frequency-dividing the clock signal; and
an electrical difference-frequency signal generator for generating an electrical difference-frequency signal by frequency-mixing the electrical offset signal and the electrical difference-frequency signal together;
wherein the electrical driving signal is generated based on a phase difference between the electrical offset signal and electrical difference-frequency signal.
In this embodiment, preferably, there are additionally provided with
(d) an electrical cross-correlated signal generator for generating an electrical cross-correlated signal from the cross-correlated light;
(e) a peak value sampler for sampling peak values of the electrical cross-correlated signal; and
(f) a display device for displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
Preferably, the peak value sampler (e) samples the peak values of the electrical cross-correlated signal to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.
In still another preferred embodiment of the apparatus according to the second aspect, in the electrical driving signal generator (a-3);
an electrical offset signal 4 is generated by frequency-dividing the clock signal;
an electrical difference-frequency signal is generated by frequency-mixing the electrical offset signal and the electrical clock signal together; and
the electrical driving signal is generated based on a phase difference between the electrical driving signal and electrical difference-frequency signal.
In this embodiment, preferably, there are additionally provided with
(d) an electrical cross-correlated signal generator for generating an electrical cross-correlated signal from the cross-correlated light;
(e) a peak value sampler for sampling peak values of the electrical cross-correlated signal; and
(f) a display device for displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
Preferably, the peak value sampler operates to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.
In a further preferred embodiment of the apparatus according to the second aspect, in the electrical driving signal generator (a-3);
an electrical offset signal is generated by frequency-dividing the clock signal;
an electrical, additional driving signal is generated by frequency-dividing the electrical driving signal; and
the electrical driving signal is generated based on a phase difference between the electrical offset signal and the electrical, additional driving signal.
In this embodiment, preferably, there are additionally provided with
(d) an electrical cross-correlated signal generator for generating an electrical cross-correlated signal from the cross-correlated light;
(e) a peak value sampler for sampling peak values of the electrical cross-correlated signal; and
(f) a display device for displaying an eye pattern corresponding to the electrical cross-correlated signal on a screen of a display device by repeatedly writing waveforms of the electrical cross-correlated signal while the electrical offset signal is used as a trigger.
Preferably, the peak value sampler operates to be synchronized with an electrical signal generated by frequency-dividing the electrical driving signal.